1. Technical Field
This invention relates to thin laminate passive electrical devices and methods of fabricating thin laminate passive electrical devices, such as, capacitors. More particularly, this invention relates to thin laminate passive electrical devices having a relatively rigid dielectric material between opposing conductors that minimizes or prevents electrical shorts between the conductors due to the fabrication process.
2. Description of Related Art
The printed circuit board (PCB) industry has a tremendous need for thin, copper-clad laminates for reducing real estate on the board and improving performance and functionality of electronic devices. The use of thin laminates as a capacitor in PCBs not only reduces PCB real estate, resulting in smaller size devices, but can also increase electrical performance of the final product. For example, an increase in electrical performance may include decoupling devices and lowering electrical noise characteristics. The challenge in the art is to make a very thin laminate devices having high electrical voltage resistance and shorting resistance as well other desired electrical, mechanical, and thermal properties. Aspects of the present invention can meet these needs by providing thin laminates for use in PCBs having a composite polymer-layer structure having improved electrical, mechanical, and thermal properties.
The disadvantages of the existing art can be illustrating with the aid of FIGS. 1-4. FIG. 1 is an exploded schematic illustration of the components that are found in a passive electrical device, for example, a capacitive laminate, according to the existing art. As shown in FIG. 1, a typical passive device may include two conductive foil, typically, copper, substrates 12 and 14 separated by a thin layer of insulating material 16, for example, a polymer, such as, an epoxy polymer. FIG. 2 is a schematic illustration of the typical lamination of foils 12, 14 with insulating material 16 under pressure and temperature resulting in a laminated passive electrical device 18.
However, close examination of the interface between the laminated foils 12 and 14, and the insulation 16 shown schematically in FIG. 3, which represents the detailed view 3 shown in FIG. 2, illustrates the recognized disadvantages of such a prior art lamination and lamination process. As shown in FIG. 3, due to the inherent unevenness or presence of asperities or peaks 20 on the surfaces of the conductive foils 12, 14, the desired separation between foils 12, 14, even with the presence of the insulation material 16, can be compromised, as indicated by the contacting peaks identified at point A. This diminished or non-existent separation between the foils 12 and 14 can be the source of electrical shorts between the foils 12 and 14 as evidence by failure of the passive device, for example, a capacitor, to provide its desired function. Accordingly, many attempts have been made in the art to minimize or eliminate contact between foils 12, 14.
One direction taken in the prior art is to provide a smooth a surface to the foils 12 and 14 as possible to minimize the number of asperities 20 that can provide potential shorting. For example, the laminates disclosed in U.S. Pat. No. 6,274,224 assigned to 3M Innovative Properties Company, provide surface finishes on conductive foils to 300 nanometers (nm) and lower. In a successful attempt to address this issue, the inventors of U.S. Pat. No. 6,693,793 of Mitsui Mining & Smelting Co., Ltd. introduced an intermediate film to minimize or prevent the potential for failure due to shorting.
Another disadvantage of prior art laminations and laminating methods can be the susceptibility to damage due to the presence of foreign matter or debris in the lamination process. Though typically practiced under extremely clean environments, for example, in “clean rooms,” the presence of minute contaminants or debris can interfere with the performance of the passive electrical device. An example of one such disadvantage is shown in FIG. 4.
FIG. 4 is a schematic illustration of a detailed view of the interface between conductive foils 22, 24 and an insulating layer 26 according to the prior art. As indicated schematically by foreign body or debris 28, during lamination, that is, under temperature and pressure, in this case, as indicated by arrows 30, the presence of foreign body 28 can result in uneven localized compression and localized deflection of one or both of the foils 22, 24. Typically, since the insulating layer 26 experiences a reduction in viscosity due to the elevated temperature at which lamination takes places, that is, at a temperature of at least 150 degrees C., under the influence of the localized pressure gradient, the insulating layer 26 may flow away from localized areas due to the uneven pressure and deflection caused by the presence of foreign body 28. As a result, there may be little or no resistance to localized deflection of the foils, which may result in undesirable reduction or elimination of the desired spacing between foils 22, 24, as indicated by the contacting peaks identified at point B in FIG. 4. Again, the diminished or non-existent separation between the foils 22 and 24 can be another source of electrical shorts between the foils 22, 24 as evidence by failure of the passive device. As will be apparent from the description below, aspects of the present invention also address this disadvantage of the prior art.